Glass-ceramic articles, i.e., articles prepared by subjecting glass articles to a controlled heat treatment to effect crystallization in situ, are well known to the art. The method for producing such articles customarily involves three fundamental steps: first, a glass-forming batch is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, third, the glass body is heated to temperatures above the transformation range of the glass in a controlled manner to generate crystals in situ. Frequently, the glass body is exposed to a two-stage heat treatment. Hence, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching or even exceeding the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained and the articles are typically more highly crystalline.
Because glass-ceramic articles are generally highly crystalline, viz., greater than 50% by volume crystalline, they are normally mechanically stronger than the precursor glass articles from which they were derived. Hence, annealed glass bodies conventionally demonstrate modulus of rupture values in the range of about 5,000-10,000 psi, whereas the glass-ceramic product will exhibit moduli of rupture over the interval of 10,000-20,000 psi. Although the latter values represent a significant improvement, numerous investigations have been undertaken to enhance the mechanical strength of glass-ceramic bodies. Two methods for achieving that goal have been utilized commercially. The first has involved applying a glaze to the surface of the glass-ceramic, which glaze has a coefficient of thermal expansion lower than that of the body. The second has comprehended subjecting the body to chemical strengthening via an ion exchange reaction. Both of those techniques are effective in increasing the mechanical strength of glass-ceramic articles, but both also have two practical disadvantages. Hence, both methods require the body to be subjected to a further process which adds cost to the product. More importantly, however, both procedures increase the internal tension in the body such that, upon breakage, the body tends to fragment into a large number of small pieces. This phenomenon is especially significant when the product is designed for consumer goods where "gentle" breakage is desired with a resultant few large pieces. Accordingly, a glass-ceramic having high intrinsic body strength would be most desirable.